This invention relates to a flat vault for tunnel type furnaces, particularly to be used for baking bricks, tiles, ceramics, and the like, or in steel mills, and particularly to a self-supporting flat vault.
It is known that tunnel-type furnaces are built in the shape of a linear tunnel on the floor of which (if the material to be baked is carried on trucks) there may be laid a rail-track, for guiding the trucks carrying the material to be baked.
A major problem to be solved in the erection of said furnaces has been the design and construction of the vault.
Known types of vaults are usually built, by a static point of view, using the so-called flat arch structure the shape of which does not require any special anchoring means, and is self-supporting.
However, it has been observed that such a kind of vault negatively affects the performance of the furnace, in that there is left useless the whole room between the upper surface of the material, usually piled on the trucks in parallelepiped stacks and the inner side of the vault. It was impossible to use the free volume left, to increase the filling of material to be baked, said volume being on the contrary full of a mass of air to be heated. Moreover said type of vault imposes limitations on the useful width of the furnace.
No attention was even paid to the idea of stacking the material of the upper layers on the truck according to a profile complementary to that of the vault, in order to increase the filling coefficient of the furnace, in that the loading operations of the trucks would come out to be too difficult and time-consuming, and the material would afterwards have to be readjusted before packaging.
Flat vaults have then been built, so that the upper level of material on the trucks was slightly lower than the height of the tunnel. The furnace could then be filled as completely as possible. Heretofore, the erection of a flat vault has been performed by linear segments of refractory material, aligned on parallel rows, transversely to the direction of the forward movement of the material to be baked, said segments being clamped to each other by means of expansion joints each row being also connected to the adjacent ones by other expansion joints.
However this kind of structure is not self-supporting, whereby each row of segments of refractory material must be provided with an associated upper metal supporting bar, parallel to said segments, and the segments themselves hang from the bar being joined thereto by means of metal tie rods. The supporting bar is anchored in a way already known, per se, to the walls of the tunnel.
A vault so constructed, while overcoming the problem of a good filling coefficient of the furnace, is however affected by several disadvantages.
A first disadvantage is due to the exceedingly high weight of the vault, which bears on the side posts or masonry of the tunnel, said posts or masonry having to be sized accordingly, and moreover to the complex static system solving the problem of supporting the vault by external supporting bars.
Another disadvantage is due to the difficulties during erection of such a vault, in that it is necessary that each segment be clamped, one at a time, to the associated tie rod, which in turn is clamped to the supporting bar.
A further disadvantage is due to the very high cost of the vault, in that the metal frames involved in a so high proportion must be thermally stable, and therefore of good quality material, for example stainless steel, or thermostable cast iron. Moreover, the most critical limitation of a vault built accordingly is found in the connection zone between the metal frames and the refractory segments, which is subjected to very high temperatures.
It is known, on the other hand, that for this type of vault, a cooling system has to be provided, essential to remove heat from metal members in order to keep said members at a suitable temperature, according to their thermal resistance. Said system must be provided outside the vault and above the vault itself, in the positions where the metal tie rods and the supporting bars have been installed. That implies difficult heat removal conditions, because the cooling air is not in direct contact with the whole metal frame. On the contrary, the metal members located in those zones where the action of the cooling air has the least efficiency, are just the ones contacting the refractory material segments, said members being therefore subjected to the highest temperatures, i.e. to the most critical operating conditions, as aforesaid.